partial class Examples
{
  public static Result LockLayer(RhinoDoc doc)
  {
    string layer_name = "";
    var rc = RhinoGet.GetString("Name of layer to lock", true, ref layer_name);
    if (rc != Result.Success)
      return rc;
    if (String.IsNullOrWhiteSpace(layer_name))
      return Result.Nothing;

    // because of sublayers it's possible that mone than one layer has the same name
    // so simply calling doc.Layers.Find(layerName) isn't good enough.  If "layerName" returns
    // more than one layer then present them to the user and let him decide.
    var matching_layers = (from layer in doc.Layers
                           where layer.Name == layer_name
                           select layer).ToList<Rhino.DocObjects.Layer>();

    Rhino.DocObjects.Layer layer_to_lock = null;
    if (matching_layers.Count == 0)
    {
      RhinoApp.WriteLine("Layer \"{0}\" does not exist.", layer_name);
      return Result.Nothing;
    }
    else if (matching_layers.Count == 1)
    {
      layer_to_lock = matching_layers[0];
    }
    else if (matching_layers.Count > 1)
    {
      for (int i = 0; i < matching_layers.Count; i++)
      {
        RhinoApp.WriteLine("({0}) {1}", i+1, matching_layers[i].FullPath.Replace("::", "->"));
      }
      int selected_layer = -1;
      rc = RhinoGet.GetInteger("which layer?", true, ref selected_layer);
      if (rc != Result.Success)
        return rc;
      if (selected_layer > 0 && selected_layer <= matching_layers.Count)
        layer_to_lock = matching_layers[selected_layer - 1];
      else return Result.Nothing;
    }

    if (layer_to_lock == null)
      return Result.Nothing;

    if (!layer_to_lock.IsLocked)
    {
      layer_to_lock.IsLocked = true;
      layer_to_lock.CommitChanges();
      return Result.Success;
    }
    else
    {
      RhinoApp.WriteLine("layer {0} is already locked.", layer_to_lock.FullPath);
      return Result.Nothing;
    }
  }
}

import rhinoscriptsyntax as rs
from scriptcontext import doc

def lock():
    layerName = rs.GetString("Name of layer to lock")

    matchingLayers = [layer for layer in doc.Layers if layer.Name == layerName]

    layerToLock = None
    if len(matchingLayers) == 0:
        print("Layer \"{0}\" does not exist.".format(layerName))
    elif len(matchingLayers) == 1:
        layerToLock = matchingLayers[0]
    elif len(matchingLayers) > 1:
        i = 0;
        for layer in matchingLayers:
            print("({0}) {1}".format(i+1, matchingLayers[i].FullPath.replace("::", "->")))
            i += 1

        selectedLayer = rs.GetInteger("which layer?", -1, 1, len(matchingLayers))
        if selectedLayer == None:
            return
        layerToLock = matchingLayers[selectedLayer - 1]

    if layerToLock.IsLocked:
        print("layer {0} is already locked.".format(layerToLock.FullPath))
    else:
        layerToLock.IsLocked = True
        layerToLock.CommitChanges()

if __name__ == "__main__":
    lock()